H5N1 avian influenza virus may become the cause of the next flu pandemic. Annual outbreaks of influenza A infection are an ongoing public health threat and novel influenza strains can emerge periodically to which humans have little immunity, resulting in devastating pandemics. The “Spanish flu” pandemic of 1918, caused by the H1N1 influenza virus, killed more than 40 million people world-wide. The origin of H1N1 may have gone directly from birds to humans, or it may have involved incubation in an intermediate host, such as the pig or another, as yet unidentified, animal host (1). Both the 1957 pandemic and the 1968 pandemic, caused by the H2N2 and H3N2 influenza viruses, respectively, likely originated as reassortments in which one or both human-adapted viral surface proteins were replaced by proteins from avian influenza strains (2).
The H5N1 virus has the ability to infect an unprecedented range of hosts, including carnivores. The first confirmed instances of AIV H5N1 infecting humans took place in 1997. Highly pathogenic H5N1 infections occurred in both poultry and humans. This was the first time an avian influenza virus transmission directly from birds to humans had been found. Thereafter, according to the World Health Organization (WHO), the total number of human H5N1 cases, since the initial outbreaks in south-east Asia which occurred in 2003, has reached 281, with 169 deaths. Indonesia reported its first human case of avian flu caused by the H5N1 virus in June of 2005. To date, it is the only country to report cases in 2007, with 81 confirmed human cases, 63 of which were fatal, as of March, 2007.
Influenza viruses are classified according to their nucleoprotein and matrix protein antigenic specificity. These viruses are categorized mainly into A, B and C serotypes, with type A having eight RNA segments that encode ten viral proteins. All known type A influenza viruses originated in birds. This category of virus can infect other species, such as horses, pigs, owls and seals, and poses a threat to humans as well (23). Influenza A virus is further divided into subtypes according to the antigenic nature of the envelope glycoproteins, hemagglutinins (“HAs”), H1 through H16, and neuraminidases (“NAs”), N1 through N9 (24, 25, 26). It is believed that proteolytic cleavage of HA protein at the HA1-HA2 junction is related to the pathogenicity in avian strain and that the presence of hydrophobic amino acids around this cleavage site are characteristic of the H5 subtype. In addition, the HA protein is believed to mediate attachments to host cell sialoside receptors and subsequent entry by membrane fusion (27), and HA protein is thought to serve as a primary target for neutralizing antibodies (26).
Testing during an outbreak of an acute respiratory disease can determine if influenza is the cause. During influenza season, testing of selected patients presenting with respiratory illnesses compatible with influenza can help establish whether influenza is present in a specific patient population and help health-care providers determine how to use their clinical judgment for diagnosing and treating respiratory illness. A rapid influenza test helps in the determination of whether to use an antiviral medication. Some tests, such as a viral culture, reverse-transcriptase polymerase chain reaction (RT-PCR) and serological testing are the routine methods, but results may not be available in a timely manner to assist clinicians (3). At present, most of the rapid diagnostic tests currently in use are monoclonal antibody-based immunoassays (3, 4, 5). Immunofluorescence (fluorescent antibody staining) is the alternative to rapid influenza diagnostic tests which can be used in many hospital laboratories and generally can yield test results in 2-4 hours. Above all, specific monoclonal antibody generation is fundamental to most currently used rapid, sensitive and cost-effective diagnostic methods.
The identification of regionally distinct sublineages has indicated that the H5N1 virus is geographically broad with great genetic and antigenic diversity. Phylogenetic analysis showing that all viruses from Indonesia form a distinct sublineage of H5N1 genotype Z viruses suggests that this outbreak likely originated from a single introduction that spread throughout the country (14, 15). It would be very useful to have available monoclonal antibodies which specifically recognize Indonesia influenza isolates. It further would be useful to have available such mAbs which also cover Vietnam and Singapore influenza isolates.